1. Field of the Invention
The present invention relates generally to the field of power supplies and particularly to power supplies capable of switching between mutually exclusive primary circuits for voltage input, thereby allowing the use of multiple and diverse power sources.
2. Technical Background
Electronic equipment is often used in critical applications where the loss of a power source results in significant impact to the user. Power supplies, which are capable of seamlessly switching to a backup power source, help to minimize this impact. For example, in the data communications field, power supplies often provide operational power to electronic equipment employed by emergency communication services, such as 911 services. In the event of failure of the main power source, it is essential that the process of receiving calls and dispatching emergency vehicles remains intact. Reliable electronic equipment powered by redundant and diverse power sources lessens the probability of a complete breakdown of the emergency process. If a blackout occurs and the main power source that drives the electronic equipment is affected, then there is a need for some type of backup power source.
In order to provide redundancy, and hence reliability, provisions are often made in the design of power supplies to support circuit powering from more than one voltage source. For example, a power supply that drives data communications equipment applications sometimes allows the equipment to be powered locally from an available power source, or powered from voltages received over a communications channel. In the event of failure, this type of power supply allows for seamless transition between the local power source and the alternative voltage received over the communication channel.
Power source redundancy can be achieved in electronic equipment by including in the equipment design more than one power supply. This type of design would include two separate and distinct transformers with two separate and distinct voltage sources. An external switch monitoring the primary input of one of the power supply inputs could provide a switching mechanism between the power supplies in the event of a loss of power. In other words, if the currently active power supply is affected, then the switch can activate the alternative supply. However, this type of backup power supply design is expensive, requiring two separate power supplies.
Redundancy is also achieved in the art with switching power supplies. Switching power supplies are generally designed to receive an alternating current (AC) source. A second power source can be used as input, such as a battery. A battery, however, outputs a Direct Current (DC) voltage. Therefore, prior to the use of the backup battery voltage, a DC to AC conversion is required. An example includes electronic equipment having an Uninterruptable Power Supply (UPS). Generally, an UPS functions primarily from local main power. However, if there is a disruption in the current flow (i.e. a blackout), then a battery is used as a backup source of voltage input to the power supply. However, this type of design requires a complicated conversion circuit.
Another solution to the redundancy approach is described in U.S. Pat. No. 5,751,564. The ""564 design is a dual input circuit power supply. Generally, the invention includes a transformer having two primary circuits corresponding to two separate and distinct power sources. The switching between the first and second primary circuit is achieved by a switching circuit that monitors the input of the first primary circuit, and switches to the second primary circuit upon a change in the current flow from the first power source. This type of design solution, however, has significant disadvantages. It is prone to ground-loop current, and hence is unreliable. The ""564 design and it""s lack of voltage isolation between power sources fails to provide adequate protection for the power source provider.
This ground-loop current disadvantage of the power supply disclosed in the ""564 design is created by the existence of the two mutually exclusive primary circuits corresponding to each power supply input. When the ground potentials are different between two physically connected circuits, voltage will flow from high potential to low potential. A higher potential in the first primary circuit and a lower potential in the second primary circuit may cause a ground-loop surge. Ground-loop surges caused by the redundant dual circuit can then cause damage to the circuitry of the power supply. Consequently, the power supply is fault prone and unreliable.
The non-isolated topology of the ""564 design fails to provide adequate protection to either power source in the event of a power supply component failure. For example, customer premise data communications equipment is powered from local mains power and uses a voltage delivered over the communications channel physical medium for backup power. Adequate. protection is a requirement in order to insure a failure of the remote end equipment does not apply a hazardous or damaging voltage to the service providing central office equipment.
Therefore, what is needed in the industry is an inexpensive dual input power supply that is reliable in light of the threat of damage caused by potential differences between multiple primary circuits and affords adequate protection to the individual power sources. In addition, the transition between the various power sources must be nearly instantaneous and seamless to the user.
One object of the present invention is to provide a dual input power supply including two mutually exclusive primary circuits. The present invention allows for multiple input sources by including in the transformer circuit multiple primary windings and a single secondary winding. The multiple primary windings correspond to the separate and distinct multiple power sources.
Another object of the present invention is to allow for the primary circuits to function without the threat of ground-loop current damage. The present invention isolates the primary circuits relating to the corresponding power source inputs. This is accomplished by including, in the power supply circuitry, optical isolators positioned such that, if a current is seeking a lower voltage potential, it is unable to travel from primary circuit to primary circuit. Data signals communicated from one primary circuit to another are converted to light pulses by the isolator; however, the optic air gap prohibits ground-loop current from traveling from circuit to circuit. This resultant isolation provides a high degree of protection for the individual power sources.
Another object of the present invention is including, in the power supply, control circuits for each primary circuit in order to allow selection of one primary circuit. In this multiple control circuit approach, each control circuit is connected so that the selection process can be achieved. In order to achieve optical isolation between the primary circuits, optical isolators must be placed in each link connecting the control circuits.
Another object of the present invention is implementing an optical feedback circuit from the output of the power supply. The optical feedback circuit further provides isolation between the primary circuits.